Graphene purely consists of carbon and has an aligned hexagonal structure formed into a honeycomb shape similar to graphite. However, graphene is formed of a monoatomic layer and thus basically has a big difference in physical property from graphite having a layered structure formed by layering numerous sheets of graphene. Unlike graphite, graphene behaves like Dirac fermions with a zero effective mass and is transparent enough to transmit more than 98% of light; it is about 200 times stronger than steel, two or more times higher in thermal conductivity than diamond, higher in electrical conductivity than copper, and higher in charge carrier mobility than silicon. Graphene has been expected to be widely applied to flexible displays, transparent electrodes, solar cells, etc. due to its excellent optical, mechanical, and electrical properties. Therefore, a lot of studies on the method for mass-producing graphene, the method for improving electrical conductivity, etc. have been conducted.
Various methods for preparing monolayer graphene or multilayer graphene including multiple layers have been reported. Particularly, a mechanical method of mechanically separating graphene from graphite, a chemical vapor deposition method of directly synthesizing graphene on a substrate, and a chemical method of oxidizing graphite to produce graphene oxide and preparing reduced graphene oxide from the produced graphene oxide have attracted a lot of attention and have been studied. The mechanical method has an advantage in that it can obtain undamaged monolayer graphene, but has a disadvantage in that it is not suitable for mass production. The chemical vapor deposition method has advantages in that it is suitable for mass production and can obtain a relatively high-quality multilayer graphene thin film, but has a disadvantage in that graphene is grown only on a specific metal used as a catalyst and the grown graphene needs to be transferred to another substrate. However, in the chemical method, graphite is oxidized in an acidic solution with addition of a strong oxidizer such as KMnO4 or H2O2 to produce graphene oxide containing oxygen, and then, reduced graphene oxide is prepared through a solution process of adding a reducing agent such as hydrazine (NH2NH2) or NaBH4 or a heat treatment in a vacuum or in a hydrogen atmosphere. The chemical method is relatively easily performed and economical, and, thus, can produce reduced graphene oxide which can be applied in various ways. However, the chemical method has a disadvantage in that graphene loses its own optical and electrical properties due to defects and nitrogen doping, etc., and, thus, cannot exhibit all of its properties. Nonetheless, studies on the process for preparing reduced graphene oxide have been continuously conducted. This is because reduced graphene oxide has enough properties to be applied to various fields such as electrodes, conductive inks, catalyst supports, etc.
Among various methods for preparing a reduced graphene oxide, particularly, a method using a liquid hydrazine is suitable for mass production and enables a graphene surface to be easily functionalized and also readily produces relatively high-purity graphene without metallic impurities. Thus, it is one of the most widely used methods (Korean Patent Laid-open Publication No. 10-2012-0039799). However, the reduced graphene oxide prepared using liquid hydrazine has a low carbon-oxygen atomic ratio of about 10 due to water contained during the process and also has a low conductivity due to a high content of nitrogen. Until now, the chemical reduction methods using hydrazine includes a solution process using a solvent containing water, and, thus, an additional dehydrating and solvent-removing process such as a high-temperature heat treatment are required. Due to that a small amount of moisture remains in the reduced graphene oxide despite of such an additional process, its durability is decreased and also, its conductivity becomes lower than that of graphene.